it is not completely clear to me whether concrete backerboard is used IN PLACE OF or IN ADDITION TO greenboard drywall, when putting tiles ?
ALWAYS, ALWAYS, ALWAYS Use ONLY cementitious backer board when installing tile in a wet environment.
Pretty much Moisture Resistant (AKA 'green board') is now obsolete. It was used for decades as a 'bathroom' sheetrock.
Essentially, what it is: regular sheetrock with a wax coated paper over the surface as opposed to regular sheetrock paper on it's surface.
(The newer materials to use are Densarmor and XP board (Purple face) for environments that may have ''high humidity'')
Wet environments like a shower or whirlpool tub enclosure base - Always use cement board under the tile.